As one means of determining the quantity of a substance contained in a sample liquid, electrochemically measuring the concentration of the substance in the sample liquid using a sensor is known. As the sensor, a sensor of a capillary type is widely used which is so designed that the sample liquid is introduced to a reagent layer by capillary action. Generally, such a capillary sensor includes a capillary having an inlet port. The capillary includes a pair of electrodes and a reagent layer laminated thereon. The reagent layer contains various reagents necessary for a certain reaction system such as a redox chemical substance or oxidoreductase depending on the substance to be measured, i.e. the object of the determination.
In determining the quantity of the substance contained in the sample liquid using such a sensor, the sensor is set to a quantitative analyzer and the sample liquid is supplied to the inlet port of the capillary of the sensor. At that time, the paired electrodes of the sensor are electrically connected to the voltage application means incorporated in the quantitative analyzer. The sample liquid travels within the capillary by capillary action to reach the reagent layer and dissolves the reagent such as a redox agent contained in the reagent layer. Part of the reagent reacts specifically with the substance in the sample liquid. Thereafter, when the quantitative analyzer applies a predetermined voltage to the reagent layer, a current corresponding to the concentration of the substance flows between the paired electrodes. The quantitative analyzer measures the current. The concentration of the substance is determined based on the measured current and a calibration curve or a calibration-equation prepared in advance.
For example, for measuring the glucose concentration in blood, use is made of a biosensor provided with a reagent layer containing glucose oxidase and potassium ferricyanide. When the blood is introduced to the reagent layer, glucose oxidase oxidizes glucose to gain an electron, whereas potassium ferricyanide receives the electron from glucose oxidase to be reduced to potassium ferrocyanide. Thus, when a predetermined voltage is applied across the paired electrodes, a current corresponding to the concentration of potassium ferrocyanide flows between the electrodes. Since the potassium ferrocyanide concentration corresponds to the glucose concentration in the blood, the glucose concentration in blood is determined by measuring the current.
As a prior art quantitative analyzer for electrochemically measuring the concentration of a substance in a sample liquid using a sensor, an automatic quantitative analyzer is known which uses a disposable sensor and utilizes a potential step method. Generally, such an automatic quantitative analyzer is designed to automatically start the measurement operation when a sample liquid is introduced to the reagent layer. Specifically, when a sensor is set to the quantitative analyzer, a predetermined voltage is applied to a pair of electrodes of the sensor, as shown in FIG. 10. When the sample liquid is introduced to the inlet port of the capillary, the sample liquid is guided to the reagent layer by capillary action, thereby changing the current flowing between the electrodes. Based on the change in the current, the quantitative analyzer detects the introduction of the sample liquid to the reagent layer and intermits the application of voltage across the electrodes (time t0). After the lapse of a predetermined period, the automatic quantitative analyzer restarts the application of voltage across the electrodes (time t2) and measures the current flowing between the electrodes. Subsequently, the automatic quantitative analyzer determines the concentration of the substance based on the measurements and a table prepared based on a calibration curve or a calibration equation, for example.
However, in such a prior art quantitative analyzer, the current flowing between the paired electrodes is utilized only for determining whether or not the sample liquid is introduced to the reagent layer, and the measurement is started without considering the amount of the sample liquid introduced to the reagent layer. Therefore, in the prior art automatic quantitative analyzer, the concentration of the substance may be determined erroneously due to the insufficiency in the amount of the sample liquid.
For example, in the case where the sample liquid is blood, even when a suitable amount of blood is supplied to the sensor, the amount of blood reaching the reagent layer by capillary action may be in sufficient due to the high hematocrit or viscosity. When the amount of blood introduced to the sensor is insufficient, the amount of blood reaching the reagent layer is accordingly insufficient even if the hematocrit or viscosity is not so high. In such cases, the prior art automatic quantitative analyzer produces measurement results including an unacceptable level of error in the measuring of the glucose concentration in the blood.
JP-A-8-502589 discloses a method and an apparatus for determining whether or not the amount of a sample liquid supplied to a test cell is sufficient by setting two threshold values with respect to a current flowing through a reaction region in the test cell. In this prior art, the first threshold value is set in advance for detecting the introduction of the sample liquid to the reaction region, whereas the second threshold value, which is greater than the first one, is set in advance for determining whether or not the amount of the sample liquid is sufficient for continuing the measurement.
JP-A-8-502589 further discloses a method and an apparatus for determining, with respect to a current flowing through the reaction region depending on the concentration of the substance to be measured, whether or not a measured current value follows the cottrell equation. This prior art method calculates the ratio of the sum of current values at a plurality of points of time after starting the measurement to one of the current values.
Similarly, JP-A-5-502727 discloses a method for determining, with respect to a current flowing through the reaction region depending on the concentration of the substance to be measured, whether or not a measured current value follows the cottrell equation. This prior art method calculates the ratio between current values at two different points of time after starting the measurement.
The present invention is conceived under such circumstances, and an object of the invention is to provide a quantitative analyzing method and a quantitative analyzer capable of determining the state of a sample liquid, particularly the sufficiency or insufficiency of the amount of the sample liquid introduced to a reagent layer of a sensor based on a measured current which depends on the concentration of the substance to be measured even when the current does not follow the cottrell equation.